Energy harvesting systems that couple solar panels with supercapacitor buffers offer an attractive option for powering computational systems deployed in field settings, where power infrastructure is inaccessible. Supercapacitors offer a particularly compelling advantage over electrochemical batteries for such settings because of their ability to survive many more charge-discharge cycles. We share UR-SolarCap-a versatile open source design for such a harvesting system that targets embedded system applications requiring power in the 1-10 W range. Our system is designed for high efficiency and controllability and, importantly, supports auto-wakeup from a state of complete energy depletion. This paper summarizes our design methodology, and the rationale behind our design and configuration decisions. Results from the operation and testing of a system realized with our design demonstrate: 1) an achievable harvester efficiency of 85%; 2) the ability to maintain sustained operation over a two week period when the solar panel and buffer are sized appropriately; and 3) a robust auto-wakeup functionality that resumes system operation upon the availability of harvestable energy after a period in which the system has been forced into a dormant state because of a lack of usable energy. To facilitate the use of the system by researchers exploring embedded system applications in environments that lack a power infrastructure, our designs are available for download as an archive containing design schematics, Printed Circuit Board (PCB) files, firmware code, and a component list for assembly of the system. In addition, a limited number of pre-assembled kits are available upon reques